There are many applications of electrically driven equipment in which suddenly applied or overhauling loads or both are encountered. One of the more common of these applications involves hoisting equipment. Hoists commonly incorporate a drive motor, a drum on which a lifting cable is wound, and a holding brake for stopping and holding the lifted load. When the holding brake is released to permit movement of a suspended load, the drive motor must immediately provide sufficient torque to maintain control over the load. If for any reason the motor fails to produce the necessary torque, the load can be dropped causing serious damage and possible personnel injury. Similarly, if the drive motor is producing the torque level necessary to control the load during a raising or lowering operation and the motor torque drops below that level, the same results may occur.
Historically, a number of different types of electrical systems have been designed for the control of hoisting machinery. The earliest of these utilized two brake. One brake was a holding brake for stopping and holding the load and usually was of a spring applied and electrically released type. The second brake was applied mechanically in the hoist lowering direction by the action of the overhauling load suspended from the hoist, in order to prevent uncontrolled lowering. In order to lower a load with this system, it was necessary for the motor to develop torque in the lowering direction sufficient to release the mechanical load brake so that it could provide lowering control. Although this system provided for safe operation, it had several serious deficiencies in that the lowering control brake was noisy, inefficient, and subject to a high degree of wear. Systems powered by direct current motors with series fields later became available in which the motor developed braking as well as driving torque This allowed a load to be lowered without resort to mechanical load brakes or other secondary braking means. Some protection against loss of load control was provided with these systems by making the release of the holding brake dependent on the existence of a certain minimum amount of motor current. The control was such that variable speeds both in hoist raising and lowering were provided.
In time, direct current power systems largely were replaced with alternating current systems. However, the use of alternating current motors with hoisting equipment has been handicapped by the fact that such motors tend to run at a speed determined entirely by the frequency of the alternating current power supply. The difficulty is compounded by the fact that the alternating current motor cannot develop braking torque when overdriven at less than its normal full speed. Thus, slow lowering speeds cannot be attained except with auxiliary or secondary braking means. With the advent of electronic and solid state power conversion, adjustable voltage control systems using direct current motors with shunt fields to power hoisting machinery from an alternating current power supply have become common. Since this type of motor can develop torque at any speed when acted on by an overhauling load, secondary braking devices are not necessary. However, the only protection against loss of motor torque during operation normally provided is through overspeed and field loss sensing devices which are utilized to cause the holding brake to be applied.
More recently, adjustable frequency drive systems for alternating current induction motors have been developed. While these systems provide some desirable performance characteristics, they have not been widely used with hoisting machinery, at least in the absence of secondary braking devices, because of their greater tendency to lose control of the load.